1. Field of the Invention
The present invention relates to a focus detecting device for use in a lens interchangeable camera such as a single lens reflex camera.
2. Description of the Prior Art
Heretofore, there are various kinds of devices which detect the focusing condition of the objective lens by means of measuring the light coming from an object to be photographed (hereinafter referred to as object light) and having passed through the objective lens mounted on a camera body. However, such devices have a disadvantage that when the brightness of the object is low, the devices cannot detect the focusing condition precisely due to the lack of the intensity of light to be measured. It may be thought that the focus detecting device itself should project a light beam to illuminate the object and become free from the above disadvantages. In the case of the lens interchangeable camera to which various kinds or types of lenses are coupled selectively, a type of light projector which projects the light beam through the lens coupled with or mounted on the camera body is favorable since no parallax will occur between the objective lens and the focus detecting device. In the case when the light beam is projected from the rear of the objective lens to an object to be photographed, any light to be projected is likely to be reflexed by the surfaces of the lens elements of the objective lens and not reach the object. Further, the light once reflected at a surface of an lens element is reflected many times by a plurality of surfaces of the lens elements to cause a veiling glare that affects the light measurement. As the veiling glare is predominantly strong in comparison with the signal light reflected from an object to be photographed, the light detector for the focusing detection will show a bad accuracy of detection or sometimes can not detect the focusing condition when the detector receives the veiling glare.